Upload
others
View
26
Download
1
Embed Size (px)
Citation preview
DESIGN OF REINFORCED
CONCRETE FOOTINGS
Dr. Zainorizuan Mohd Jaini
Faculty of Civil and Environmental Engneering
Universiti Tun Hussein Onn Malaysia
86400 Parit Raja, Batu Pahat, Johor, Malaysia
Semester 2 2017/2018
Introduction
2
From Soil Investigations To Foundation Design
Introduction
3
❑ Foundation is the part of a structure which transmits the loads
from the structure to the underlying soil or rock.
❑ It is usually placed below the surface of the ground. All soils
compress noticeably when loaded and caused the supported
structure to settle.
❑ Two essential requirements in the design of foundation are:
i. The total settlement of the structures shall be limited to a
tolerably small amount
ii. The differential settlement of various parts the structure
shall be eliminated as nearly as possible
❑ Requirement in EC2:
- Design is similar as slab
- Shear checking for vertical shear at 1.0d, punching shear at
2.0d and punching shear at column perimeter.
Introduction
4
Introduction
5
The Lotus Riverside, Shanghai, June 2009: Foundation
design, geotechnical or construction failure?
Introduction
6
It is perhaps the ultimate example of
foundation failure - a 100% loss of the
structure. What happened?
The broken concrete pilings jutting out from the base. Concrete foundation pilings
breaking?
Classification of Foundation
❑ Classification of reinforced concrete foundation:
❑ Factor of selection: (i) the soil properties and conditions, (ii) thetype of structure and loading, (iii) the permissible amount ofdifferential settlement.
❑ The choice is usually made from experience but comparativedesigns are often necessary to determine the most economical
7
Simple/Shallow Deep
- Isolated or pad footing
- Combined footing
- Raft foundation
- Strip footing
- Strap footing
- Pile foundation
Classification of Foundation
8
Pad footing
Shallow foundation
Pile footing
Deep foundation
Classification of Foundation
9
Pad footing Strip footing
Classification of Foundation
10
Strap footing
Classification of Foundation
11
Raft footing
Classification of Foundation
12
Pile foundation
Design Consideration
❑ The major design considerations in the structural design of a
footing relate to flexure, shear, bearing and bond.
❑ In these aspects, the design procedures are similar to those for
beams and two way slabs supported on columns.
❑ Deflection control is not a consideration in the design of footing
which are buried underground (and hence no visible).
❑ However, control of cracking and protection of reinforcement by
adequate cover are important serviceability considerations,
particularly in aggressive environments.
❑ Limit the crack width to 0.3 mm in a majority of footings.
13
Serviceability Limit
StateUltimate Limit State
Size, thickness Design, Checking
Thickness and Size
❑ Thickness and size of footing:
▪The area at the base of the footing is determined from the
safe bearing capacity of the soil.
▪The thickness of footing is generally based on consideration
of shear (predominate) and flexure, which are critical near
the column location. The minimum effective depth of pad:
14
Soil bearing capacityk kG Q W
Area+ +
=
max
Ed
Rd o
Nd
v u=
Thickness based on shear criteria, where:
NEd=Ultimate vertical load=1.35Gk+1.5Qk
vRdmax=0.5vfcd=0.5[06(1-fck/250)](fck/1.5)
uo=Column perimeter
Soil Pressure
❑ The distribution of soil pressure under a footing is a function of
the type of soil and relative rigidity of the soil and the footing.
❑ For design purposes, it is customary to assume the soil
pressures are linearly distributed, such that the resultant
vertical soil force is collinear with the resultant downward force.
❑ For pad footing:
15
EdNP
A=
( )N W yP M
A I
+=
Flexural Design
❑ The footing base slab bends upward into a saucer-like shape
on account of the soil pressure underneath it.
❑ The critical section for bending is at the face of the column.
❑ The moment is taken on a section passing completely across
the footing and is due to the ultimate loads on one side of the
section. The moment and shear forces should be assessed
using STR combination 1:
16
1.35 1.5k kN G Q= +
Flexural crack forming near the column-to-spread footing
Shear Resistance
❑ Footing may fail in shear as beam shear or punching shear at
the location shown in the figure below:
17
Location of critical shear
section and perimeter
Shear Resistance
❑ Footing may fail in shear as beam shear or punching shear at
the location shown in the figure below:
18
Vertical shear Punching shear
The critical section for vertical
shear is at a distance d from the
face of the column.
The vertical shear force is the sum
of the loads acting outside the
section.
If VEd < VRd,c → no shear
reinforced is required.
VEd = the design shear force
VRd,c =the concrete shear
resistance
The critical section for punching
shear is at the perimeter 2d from
the face of column. The punching
shear force is the sum of the loads
outside the critical perimeter. The
shear stress is vEd = VEd/ud where
u is the critical perimeter.
If vEd < vRd,c → no shear reinforced
is required.
The maximum punching shear at
the column face must not exceed
the maximum shear resistance
VRdmax.
Shear Resistance
❑ Punching shear resistance can be significantly reduced in the
presence of a coexisting bending moment, MEd, transmitted to
the foundation.
❑ To allow for the adverse effect of the moment, which gives rise
to a non-uniform distribution of shear around the control
perimeter Cl.6.4.3(3) of EC2 gives the design shear stress to
be used in punching shear.
where;
19
EdEd
i
Vv
u d=
1
1
1 Ed
Ed
M uk
V w = + Factor used to include the effect of
eccentric loads and bending moments
k = coefficient dependent on the ratio between the column
dimension (c1 and c2).
Shear Resistance
20
c1/c2 ≤0.5 1.0 2.0 ≥ 3.0
k 0.45 0.60 0.70 0.80
u1= the length of basic control perimeter
w1= function of the basic control perimeter corresponds to the
distribution of shear
w1= 0.5c12+c1c2+4c2d+16d2+2πdc1
Shear distribution
due to an
unbalanced moment
Shear Resistance
❑ Vertical shear at 1.0d from column face is based on shear
resistance, VRd,c > Ved
❑ Punching shear at 2.0d from column face based on punching
stress:
❑ Punching shear at column perimeter is based on maximum
shear resistance
21
= 1/3 3/2 1/2
, 1[0.12 (100 ) ] [0.035 ]Rd c ck ckV k f bd k f bd
= 3/2 1/2
min [0.35 ]V k f bd
,min
,
Ed punching
Rd c Ed
VVv v
ud ud= =
,max 0.5 0.6 1250 1.5
ck ck
Rd Ed
f fV ud N
= −
Crack Control
❑ Use similar rules as for beam in Cl.9.3 of EC2 and Cl.7.3,
Table 7.2N and 7.3N
❑ Steel stress for limiting crack width, wmax=0.3mm
or
❑ For detailing requirement, maximum allowable spacing is
250mm
22
0.3 1
1.15 1.35 1.5yk k k
sk k
f G Qf
G Q
+ = +
0.3435
1.35 1.5sreqk k
sk k sprov
AG Qf
G Q A
+ = +
Detailing
23
Column
Footing
Spacers
Detailing
24
Footing Design
25
Example 4.1:
Pad footing under axial
A rectangular pad foundation is required to support a singlecolumn transferring an axial service load which consist of 600kNpermanent load and 450kN variable load. Using the dataprovided, determine the suitable size of the footing and designthe required reinforcement.
Design data:fck = 25 N/mm2fyk = 500N/mm2Soil bearing capacity = 200N/mm2Column dimension = 300mm x 300mmDesign life = 50 yearsExposure class = XC2
Example 4.1
Example 4.1
Example 4.1
eff
Example 4.1
Example 4.1
Example 4.1
Example 4.1
Example 4.1
Example 4.1
Footing Design
35
Example 4.2:
Pad footing under axial +
uniaxial-moment
Design a rectangular pad footing of uniform thickness for areinforced concrete column of size 300mm x 300mm andcarrying axial load of 1500kN and ultimate bending moment of50kNm.
Design data:fck =25 N/mm2fyk =500N/mm2Soil bearing capacity = 200N/mm2Column dimension = 300mm x 300mmNominal cover = 40mm
*Use a factor of 1.40 to change the design load to service load.
Example 4.2
Example 4.2
Example 4.2
Example 4.2
eff
eff
Example 4.2
Example 4.2
Example 4.2
Example 4.2
Example 4.2
Example 4.2